1. Field of the Invention
This invention relates to a reciprocating optical modulator which is capable of effecting optical modulation with a high-frequency radio signal in a simple structure and permitting downsizing as well.
2. Description of the Prior Art
JP-A 2000-6275 discloses an optical modulator which, by means of a device provided with a structure for introducing a laser beam into a kind of light reflecting resonator and causing the light being reciprocated through the interior thereof to be modulated up to a plurality of reflections, thereby obtaining a higher-order sideband and with a band-pass filter for passing out of the higher-order sideband the part thereof exceeding the expected order of sideband, is enabled to obtain a light modulated in intensity with a frequency which is integer times the frequency of the input high-frequency electric signal. It further discloses a structure which has an optical amplifier disposed in the reflecting resonator.
A laser device having a light path, a reflecting layer and a reflecting grating range provided in an erbium (Er)-doped lithium niobate (LiNbO3) crystal as illustrated in FIG. 6 herein is disclosed in “Integrated optical Ti:Er:LiNbO3 distributed Bragg reflector laser with a fixed photorefractive grating,” Ch. Becker, et al., OPTICS LETTERS, Vol. 23, No. 15, 1194–1196, Aug. 1, 1998.
A laser device which is furnished with an optical resonator formed of a region interposed between two reflecting grating regions adopted in the place of the reflecting layer mentioned above as illustrated in FIG. 7 herein is disclosed in “Single-frequency Ti:Er LiNbO3 distributed Bragg reflector waveguide laser with thermally fixed photorefractive cavity,” B. K. Das, et al., Appl. Phys., B73, 439–442 (2001).
Further, an optical modulator which has a Mach-Zehnder optical interferometer formed on a lithium niobate (LiNbO3) crystal has been well known already in the art.
The optical modulator disclosed in JP-A 2000-6275 is capable of obtaining a modulated light including a high-order sideband. The strength of the output produced by this modulator, however, tends to decrease suddenly in accordance as the order is heightened. With a view to exalting the output strength, therefore, a structure using an optical amplifier is disclosed. Generally, for the purpose of enhancing the response speed, it is necessary to shorten the length of the light path for reciprocation of the light. Since the structure which permits downsizing is not disclosed, it is still impossible to effect modulation with a modulating signal of a broad bandwidth. Though the laser device which is disclosed in the prior art of Ch. Becker, et al. or B. K. Das, et al. is usable as an oscillator or an amplifier, it cannot be used as an optical modulator. Further, the Mach-Zehnder optical modulator formed on the lithium niobate (LiNbO3) crystal incurs difficulty in obtaining a higher-order sideband with a high-frequency electric signal of small amplitude.
This invention has been initiated in view of the state of affairs described above and is aimed at providing a reciprocating optical modulator which, in spite of effecting modulation with a modulating signal having a broad bandwidth, is capable of easily modulating light with a high-frequency radio signal in a simple structure and permitting downsizing as well.